Recently, keen attention is being paid to the problem of global warming due to GHG, such as CO2, CH4, N2O or the like. To determine a discharge/leakage quantity of various GHG from the ground or industrial plants or to determine a CO2 absorption quantity by a forest is becoming more and more important.
A most simplified method to determine the gas discharge (flux) quantity per unit area from the ground will be described with reference to FIG. 15A. In this method, a container 101 having a small hole 102 is placed so as to cover the ground 100a and a concentration of a measuring object gas in an initial state in the container 101 is measured. After passing of a predetermined time, the gas concentration is again measured. Thus, by the concentration difference and ground covering area/volume of the container, the gas flux quantity can be assessed. In the figure, numeral 104 designates a gas collector and numeral 106 an analyzer.
Also, forest CO2 flux measurement has recently begun to be actively carried out in many places. As shown in FIG. 15B, an observation tower 91 is installed in a forest 99. A current meter 51 having a good time-wise responsiveness (response ability) and a CO2 densitometer 93 are mounted on the tower 91 so that atmospheric air observation is carried out. Results of the measurements are analyzed by the eddy correlation method to thereby obtain the forest CO2 flux quantity (that is, the CO2 absorption quantity by the forest). For example, the inventors here have heretofore reported continuous observations of CO2 flux, as mentioned in Non-patent Document 1 below.
More concretely, as shown in FIG. 15B, as a current meter 51 for measuring a wind velocity, an ultrasonic current meter with a very high time-wise responsiveness is generally used. As to the CO2 concentration measurement, while it is usual to use a closed path type CO2 densitometer 96 using a sampling pipe 95, an open path type CO2 meter, as shown by the CO2 densitometer 93, with a high time-wise responsiveness using an infrared ray source (the measuring length is 1 m or less) also has recently begun to be used. In the figure, numeral 90 designates an observation room and numeral 19 an analyzer.
Moreover, if not a measurement of the gas flux itself, a regional momentum flux measuring technology using a laser has been developed and application thereof to the forest measurement is proceeding, wherein the regional momentum flux is defined as vertical directional transport properties of an atmospheric air mass (average density) being multiplied by a horizontal directional velocity component. This measuring technology, using a scintillation method, will be described. As shown in FIG. 15C, two observation towers 91, 92, being kept away from each other, are installed in the forest 99. A scintillation measuring unit 70 is mounted in a light source part 111 provided on one of the towers 91 and two laser beams are radiated therefrom so as to be transmitted above the forest 99 and received by a light receiving part 112 provided on the other of the towers 92. At the light receiving part 112, time-wise changes of respective laser transmission factors (scintillation) are measured. In the figure, numeral 90 designates an observation room, numeral 121a demodulator and numeral 122 an analyzer.
The basic construction of this prior art system comprises, as shown in FIG. 15D, a pair of scintillation measuring laser oscillators 113, 114 on the tower 91, a pair of light receivers 115, 116 on the tower 92 and the analyzer 122 provided in a measuring room 123. Two laser beams 113a, 114a transmitted through a measuring region 100 are received by the light receivers 115, 116, respectively, so that received light signals S101, S102 are sent to the analyzer 122. At the analyzer 122, an analysis 132 of variance and covariance is first carried out in order to determine an atmospheric turbulence state on the optical path (that is, an optical path turbulence analysis 131) and then a dissipation factor ε of kinetic energy or heat is obtained by an analyzing method 133 using the Monin-Obukhov similarity law (herein referred to as the MOS law). Also, a momentum flux or sensible heat flux 134 (including a latent heat flux also according to the case) is obtained.
By the way, it is generally known that, in the atmospheric boundary layer, turbulences are generated due to frictional actions and thermal actions on the ground surface and thus the upward transportation of various physical quantities are dominantly governed by turbulence transportation. According to the MOS law, it is shown that various statistical quantities of atmospheric variables in this region (average values, variances, covariances, spectra, etc.) become universal functions relative to z/L (z is a measuring height, L is a Monin-Obukhov length). Hence, in case this similarity law holds good, the atmospheric turbulence state (that is, in this case, the atmospheric density turbulences or the secondary density structure function Dn2 corresponding to time-wise changes of the laser transmission factor) is measured and, based on the MOS law, the measurement results are sequentially analyzed (that is, the atmospheric turbulence state→kinetic energy spectra→energy dissipation factors) so that the momentum flux is obtained.
In order to obtain the momentum flux in this way, it is assumed that the MOS law is applicable to the portion above the forest and, using the method mentioned in Non-patent Document 2, the atmospheric turbulence state is analyzed by the laser scintillation state so that the momentum flux quantity on the optical path is obtained (scintillation method). Steps of this method are shown in FIGS. 16A and 16B.                Non-patent Document 1: “Introduction of CO2 flux continuous observations in birch forests of the east foot of Mt. Asama” by Nakaya, O. et. al., 2002 CGER Flux Research Meeting (14 Nov., 2002), page 58.        Non-patent Document 2: “A displaced-beam scintillometer for line-averaged measurements of surface layer turbulence” by Thiermann, V., The 10th Symposium of Turbulence and Diffusion, 29 Sept.-2 Oct., 1992, Portland, Oreg., published by the American Meteorological Society, Boston, Mass., pages 244 to 247.        
In measuring the gas flux using the above-mentioned prior art methods, however, there are shortcomings as follows:
(1) The gas densitometer of the state of the art does not necessarily satisfy the necessary conditions of the flux measurements.
For the gas concentration measurements used for the flux measurements, such as the forest CO2 absorption measurements or the like, except the measurements considering no time-wise change of the flux quantity, as first shown as the prior art, the following characteristics are needed:
(i) High responsiveness
In order to detect the flux by the eddy correlation method, as quick a responsiveness (response time) as possible is demanded.
(ii) No influence by concomitants
In order to detect micro-components, it is demanded that there is given no influence by substances other than the object gas to be measured.
(iii) Measurement stability
As long time continuous measurements are needed, measurement stability is demanded.
That is, in the closed path type gas densitometer 96 of the sampling method that is generally used, there is caused a measurement delay or dilution effect for a structural reason. Hence, there is a problem in the responsiveness (response time).
Also, as the concomitants, such as H2O and solid particles, give influences, a pre-treatment (dehumidifying, dedusting) is always needed and this makes enhancement of the responsiveness (response time) difficult.
Also, in the open path type gas densitometer 93 that has begun to be gradually used for the purpose of improving the responsiveness (response time), an infrared ray source having a large oscillation width is used as the light source. Hence, there is easily given a large influence by the concomitant gas, especially by H2O. Also, there is a problem in the measuring stability for reason of the light source.
(2) Regional continuous gas concentration measurements are difficult.
That is, in the presently used closed path type gas densitometer 96, the measuring range is limited to the region in the vicinity of the sampling position. Also, in the open path type gas densitometer 93, because of the light source problem, the measuring length thereof is at most 1 m or less. Hence, by the prior art systems, measurements of 1 m or more or, for example, measurements of regional gas concentration changes of such size as 10 m, 100 m or 1 km are difficult.
Even by the prior art systems, if a multiplicity of arrayed measuring devices are used, regional gas concentration measurements will be theoretically possible. Nevertheless, if a multiplicity of measuring devices are arranged, the existence itself of these devices becomes an obstacle that will change the state of the measuring region (concentration, flux or the like). Thus, an accurate regional flux measurement will be impossible.